1. Field of the Invention
This invention relates generally to satellite-based navigational signal receivers. More particularly, it relates to a system, method, and apparatus for fast initialization of such receivers in the cold start mode.
2. Description of the Related Art
There are several satellite-based radio navigation systems currently in operation or development—the Global Positioning System (GPS), the Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), and an European satellite navigation system called GALILEO. The GPS is built and operated by the United States Department of Defense (DoD). The official name is NAVSTAR, an acronym for NAVigation Satellite Timing and Ranging. These systems permit a user with an appropriate satellite signal receiver to determine his or her position with respect to the Earth.
The constellation consists of 24 operational satellites. Due to the presence of spare satellites in the GPS constellation, it is possible to have more than 24 operational satellites.
These satellites are positioned in six different orbital planes such that at any time a minimum of six and a maximum of eleven satellites are visible to any user on the surface of the Earth, except in the polar region. The satellites operate in circular 20,200 km (10,900 nm, or about 12,000 miles) orbits at an inclination angle of 55 degrees and with a 12-hour period. The position is therefore the same at the same sidereal time each day, i.e., the satellites appear 4 minutes earlier each day.
Each satellite contains at least one atomic clock and transmits an accurate time and position signal referenced to the atomic clock. Each satellite transmits on two L-band frequencies: L1=1575.42 MHz and L2=1227.6 MHz. Three sets of pseudo-random noise (PRN) ranging codes are in use: the coarse/acquisition (C/A) code, the precision (P) code, and the Y-code.
The C/A code set has a 1.023 MHz chip rate, a period of 1 millisecond (ms) and is used in non-military positioning and to acquire the P-code. The C/A code is available on the L1 frequency. The P code has a 10.23 MHz rate. The P-code is available on both L1 and L2 frequencies. The Y-code is used in place of the P-code whenever the anti-spoofing (A-S) mode of operation is activated. All GPS satellites transmit on the same frequencies, L1 and L2, but with different individual code assignments.
More specifically, each satellite transmits a navigation message containing its orbital elements, clock behavior, system time, and status messages. An almanac that gives the approximate data for each active satellite is also provided. An almanac allows the user to generate the visible satellite list during hot start.
Generally, a GPS receiver locks onto the GPS signal and extracts data contained therein. With signals from a sufficient number of satellites, the GPS receiver can triangulate its position, velocity, and time.
A typical GPS receiver comprises an antenna, processors, and a memory. The GPS receiver may have time, position, and almanacs stored in the memory. In this case, the receiver can use the stored information to compute the visible satellites and lock onto those satellite signals in a relatively short time. This type of initialization of the receiver is called hot start. Most of the GPS receivers are initialized in the hot start mode.
In cases where the receiver has partial, incomplete information on the position, time, and almanacs, the receiver may be initialized with a warm start. In cases where the receiver has no prior data on the position, time, and almanacs, the receiver must search all of the satellites present in the constellation. This is called cold start.
In the cold start mode, a conventional receiver may automatically select a set of satellites and dedicate an individual tracking channel to each satellite, to search the Doppler range frequency for each satellite in the set. If none of the selected satellites is acquired after a predetermined period of time (time-out), the receiver then selects a new search set of satellites. This process is repeated until the sufficient number of satellites are acquired. The typical Time-To-First-Fix (TTFF) is about 45 to 100 seconds. As satellites are acquired, the receiver downloads and stores ephemeris and almanac data.
U.S. Pat. No. 6,184,824, issued to Bode et al., discloses a method for initialization of a receiver. For a cold start, satellites positioned on two orbital planes that are as close as possible to perpendicular to one another (see FIG. 1) are selected for the search sets. After the first signal is acquired, the receiver downs and stores ephemeris from the detected satellite, which takes more than 12 minutes. Ephemeris data are part of the satellite data message and contain current satellite position and timing information. The receiver then calculates from the ephemeris the current occupancy of the various orbital planes by the satellites and generates a list of applicable search sets, which is another time consuming process. Since the ephemeris data are valid for several hours, the stored information must be updated periodically.
Clearly, there is a need in the art for a new system, method, and apparatus that provides fast initialization of satellite-based navigational signal receivers in the cold start mode. The present invention addresses this need.